----- Impacts of climate change:
Temperature and precipitation --------
Global warming is underway. The global mean temperature has risen about 0.7 °C to 0.8 °C on
average above pre-industrial leve...
ls. These observed changes in climate have already had an influence on
human and natural systems (IPCC, 2007b). The greatest warming over the past century occurred at high
latitudes, with a large portion of the Arctic having experienced warming of more than 2 °C.
The projected large increase in global GHG emissions in the Baseline is expected to have a significant
impact on the global mean temperature and the global climate. The Intergovernmental Panel on Climate
Change’s Fourth Assessment Report (IPCC, 2007a) concluded that a doubling of CO2 concentrations from
pre-industrial levels (when they were approximately 280 ppm) would likely lead to an increase of
temperature somewhere between 2.0 °C and 4.5 °C7 (the so-called climate sensitivity. However, a
growing number of authors suggest that climate sensitivity values above 5 °C, such as 8 °C or higher
cannot be ruled out, which would shift even higher the estimated temperatures increase for existing
emissions level (Meinshausen et al. 2006; Weitzman, 2009).
Under the Outlook Baseline scenario, the global concentration of GHGs is expected to reach
approximately 685 ppm CO2-equivalent (CO2e) by mid-century and more than 1 000 ppm CO2e by 2100.
The concentration of CO2 alone is projected to be around 530 ppm in 2050 and 780 ppm in 2100
(Figure 3.9). As a result, global mean temperature is expected to increase, though there is still uncertainty
surrounding the climate sensitivity. The Outlook Baseline scenario suggests that these GHG-concentration
levels would lead to an increase in global mean temperature at the middle of the century of 2.0 ºC-2.8 ºC,
and 3.7 ºC-5.6 ºC at the end of the century (compared to pre-industrial times). These estimates are roughly
in the middle ranges of temperature changes found in the peer-reviewed literature (IPCC, 2007b).